This disclosure is directed to a thermal neutron porosity logging apparatus. This is a tool adapted to be lowered on an armored logging cable into a well bore, either cased or open hole. The device is enclosed in a sonde suspended on the logging cable. The sonde is lowered to a depth whereupon thermal neutron flux can be measured by detectors within the sonde.
A thermal neutron flux measurement is dependent in part on formation lithology, the nature of the fluid which fills the pores of the formation, and formation porosity. While these factors influence thermal neutron flux in a manner which enables the formation porosity to be determined (knowing the nature of the fluid and formation lithology), there are other factors which detract from use of thermal neutron flux measurements to obtain formation porosity. As an example, the borehole geometry and the positioning of the detector in the borehole are factors which alters the neutron flux rate. Moreover, the fluid that fills the borehole may also influence the neutron flux, either increasing or decreasing the count rate depending on the fluids thermal neutron capture cross section. Thus, the nature of the fluid and the volume of fluid observed by the neutrons alters the measured flux rate. The borehole geometry and the nature of the fluid are sources of correction factors necessary to obtain a useful observed count rate for determining a corrected form of formation porosity.
If the amouth of fluid interposed between the detector and the formation is reduced, variations arising from fluid capture cross section are thereby reduced. Further, more accurate measurements of neutron flux absent borehole geometry alterations in the measured value can be obtained by decentralizing the detector. It is desirable therefore to decentralize the detector within the sonde which encloses the detector and the associated electronic equipment that goes with it. Within the sonde, the detector is thus located as near as possible to one side and a decentralizing spring is located on the exterior opposite the detector. In typical installations, two detectors are used and they are located at a common aligned position to the side, both opposed by the decentralizing spring.
This embodiment thus contemplates the decentralizing of detectors and positioning an encircling shield for reducing thermal neutrons from approaching the detectors. That is, the shield is located on the side of the detectors facing the decentralizing spring. Upon decentralization of the detectors, the shielding protects against the neutron flux subject to the greatest influence by borehole geometry and fluid capture cross section.
Thus, the shielding reduces the impact of the borehole fluid and geometry on the backside of the detectors. In that sense, the shield collimates the detector in preference to thermal neutrons impinging from the formation, and prevents counting of thermal neutrons impinging from the borehole on the backside.
The foregoing in very general terms sets forth the nature of the present apparatus, but the apparatus is more readily understood on reading of the below written specification in conjunction with the drawings.